Frequency-modulated picture transmitter



W- 15, 1949 L. A. THOMPSON 2,488,517

FREQUENCY MODULATED PICTURE TRANSMITTER Filed April 22, 1944 2Sheets-Sheet l TRHNSMITTE /6 awn P475 F74 re? JNVENTOR.

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UNITED STATES PATENT. OFFICE Y 2.4mm

FREQUENCY-MODULATED rrc'runn TRANSMITTER ration of Delaware ApplicationApril 22, 1944, serial No. 532,252

2 Claims. 1

Y mit pictures by radio and one adapted for radio transmission may noteasily be adapted to transmit over a telephone line or the like. v

The present device contemplates a means whereby the standard picturetransmitter designed to transmit pictures by wire may be used totransmit pictures by radio. v

It is also well known that pictures in order to be reproduceable withfidelity when transmitted by wire rely to a considerable extent on theperfection of the line connection between the transmitter and thereceiver. That is, during transmission, variation in signal, line noisesand other disturbances are usually apparent in the final picture and insome instances have such effect thereon as to make the pictureillegible. It is particularly desirable in picture transmission fornewspaper reproduction that as good a picture as possible be obtainedbecause other losses occur during the reproduction which, if coupledwith the losses taken during transmission, may make thepicture so poorthat it cannot be used.

By my present invention I have provided an improved method, andapparatus for the transmission of pictures either over the wire or radiowhereby the picture is substantially unaffected by line noises, changesin volume and in the case of radio transmission, by static, fading andthe' like.

In carrying out the invention I contemplate using a standard picturetransmitter and a standard picture receiver, and the apparatus about tobe described comprises a converter for use during transmission and aninverter for use In other words, one complete apparatus for thetransmission of pictures would include the transmitter embodying theinvention in combination with a receiver embodying the invention toprovide a complete system.

Then finally there'is the provision of the converter itself as aseparate instrument and the inverter as a separate instrument as well asthe aforementioned combination of the two.

A brief description of the function of the apparatus will assist in anunderstanding of the apparatus itself hereinafter describedspecifically.

To begin with the picture transmitter may be of the character describedin my Patent No. 2,284,027 of May 26,1942. As described in the abovepatent it contemplates scanning a picture to provide a signal in theoutput of the machine which may be an 1800 cycle signal that varies instrength or amplitude in accordance with the reflection of light fromthe picture. This may also be considered for the purpose of explanationas an 1800 cycle amplitude modulated signal, the amplitude modulationsof which carry the intelligence. Ordinarily this signal is sent from thepicture transmitter to the picture receiver over the telephone lines orby means of land line connections. In the receiver the signal is used toreproduce the picture. During the process of transmission an amplitudemodulated signal sometimes accumulates undesirable components due toline noises or increases and decreases in signal strength all of whichaffects the desired modulation component and which results in defects inthe finished picture.

Therefore, instead of sending the signal out on the lines in the form ofan amplitude modulated wave, as it comes from the transmitter, I pass itthrough a converter. In the converter I amplify the signal to apredetermined level. Next the signal is demodulated. Then I utilize thedemodulated signal to control a variable frequency oscillator. Thevariable frequency oscillator and a fixed frequency oscillator are each'mitter or may besent by wire if desired without any change. Inthe case atransmitteris modulated it may be either a so-called amplitude modulatedtype transmitter or it may be a frequency modulated type of transmitter.

Certain improvements in methods and means for providing the signal inthe converter will also hereinafter more clearly appear in the morespecific description.

The signal after being transmitted by radio is received byany suitabletype of receiver depending on the final type of transmitter used. Duringthe transmission it" may be effected in many ways. It may accumulatestatic, noise and/or fading all of which under ordinary circumstancesmight be sufilcient to render the ordinary amplitude modulated signalsubstantially useless for high fidelity reception of picture.

After being received it is amplified together with the above mentionedundesirable defects and then passes through the inverter. In theinverter it passes through a cascade limiter which very greatly limitsthe signal, taking off all amplitude modulations. Next it is reconvertedto an amplitude-frequency modulated hybrid signal, then rectified andpassed through a filter. Then this signal is combined with an 1800 cyclecarrier. The signal has now become an 1800 cycle constant frequency,amplitude modulated signal which may be supplied to the standard picturereceiver with a result, that the picture is substantially the same as itwould have been had the facsimile transmitter been directly connected tothe facsimile receiver. Thus the signal is handled in such a manner thatthe usual losses which occur in transmission do not affect it.

It is therefore among the objects of my invention to provide a converterfor use in conjunction with a picture transmitter which enables thepicture signal to be converted for transmission by to the input. Thereactance tube I4 is directly means other than the ordinary telephoneline.

Another object of my invention is to provide an apparatus where apicture signal such as is received by radio transmission may be invertedto enable a conventional picture receiver to reproduce the same.

Another object of my invention is to provide an improved method oftransmitting pictures or intelligence which is substantially unaffectedby static, fading or the like.

Another object of my invention is the provision of an improved frequencymodulation system and method 01 operation thereof.

Another object of my invention constitutes the provision of an improvedmethod and apparatus for inverting a signal.

Other objects of the invention are the improvements of circuits embodiedin the invention and includes an improved limiter circuit, an improveddemodulation circuit and an improved signal reconstruction circuit.

Still other objects of the invention and the invention itself willbecome more apparent from the following description of a specificembodiment of the apparatus used to transmit and receive a signal whichapparatus is illustrated by the accompanying drawings and forms 'a partof this specification.

In the drawings:

Fig. 1 is a schematic drawing showing a complete transmission systemconstructed according to my invention;

Fig. 2 is a similar view of a complete receiving system for receivingand reproducing the signals from the apparatus of Fig. 1,'and

Fig. 3 is a simplified view of a portion of the circuit of Fig. 2.

One aspect of the invention includes the combination of the elements ofFigs. 1 and 2 to provide a complete system for the transmission ofintelligence. Referring now to the drawings throughout which like partshave been designated by like reference characters. In Fig. 1 I haveshown at ID a picture transmitter which may be of the character such asis illustrated and described in my patent herelnbefore mentioned. Theinvention is by no means limited to this precise form of transmitter,however, this type being shown merely for the purpose of explanation.The output from the transmitter II is connected directly to the input ofan amplifier II where the amplitude of the signal is increased to thedesired value, the amplifier being provided with a gain control H. Thesignal in the putput of the amplifier II, in case the transmitter of myprevious patent is used, will be an 1800 cycle signal having amplitudemodulations which vary in accordance with the variations in reflectiveproperties of the picture being transmitted. In other words theintelligence is contained in the modulation envelope of the signal.

A full wave diode detector I2 is connected in the output of theamplifier II. The carrier wave is rejected and the modulation componentis passed through a filter ii. The filter l3 removes any of the carrierwhich may appear in the output of the detector l2; an output, which nowcomprises the clean modulation component, is applied to the inputcircuit of the tube II which includes at least a grid and a cathode.This tube acts as a direct current amplifier and is commonly known as areactance tube. The output from the tube is in substantially directproportion coupled to and feeds the signal to an electron coupledoscillator IS.

The signal is used to control the oscillator Ii, the output of which isconnected to the input circuit of a mixer tube It. The frequency of theoscillator I5 is determined by the signal and is therefore of variablefrequency when a signal of varying amplitude is fed into it by thereactance tube. Also connected to the mixer tube I6 is a fixed frequencyoscillator H. The two signals are mixed in the mixer and in the outputthere is a composite signal comprising the variable frequency signal,the fixed frequency signal, the diflerence between the two signals, andthe addition of thetwo signals. In this case I am interested in thedifference between the two signals.

Referring back to the variable frequency oscillator, the manner ofvarying the frequency is of particular interest. It should be noted thatthe anode supply voltage for the reactance tube I4 is fed through theresistance 20 to the junction of the split inductances Zl-Z! which arethe inductive components of the resonant circuit which comprises the twoinductances' and the capacity 23. It will be noted that the capacity IIincludes a main condenser and a trimmer as is common practice. From theinductance 2| the anode supply is through the resistor 24 to the anodeof the tube M.

It will also be noted that in the oscillator circuit, this being anelectroncoupled oscillator, the second grid 26 serves as the anode, andthat the anode supply voltage is through the lower inductance 22.Therefore, it will appear that the reactance tube l4. and the anode ofthe oscillator tube l5 both get their anode supply voltages through thevoltage dropping resistor 20. when a signal appears on the grid of thetube II, that tube starts to draw current in the plate circuit. Sincethis current is through the inductance II it causes the reactance ofthat inductance to vary according to the current drawn reducing thereactance and increasing the frequency. Therefore, if the frequency ofthe oscillator circult of tube I5'is adjusted to 100 kc. with no signalon the reactance tube and the signal is applied to the reactance tubeIt, the eflfect of the reactance tube on the oscillator circuit is suchas to cause the frequency in this circuit to increase. Another importantfeature of this part of the circuit resides in the fact that the voltage-on anode 26 of the oscillator tube is supplied through the droppingresistor 20. When the reactance tube is operating with a signal on thegrid, the current drawn in the plate circuit of the tube It is throughthe resistance 20. This causes a voltage drop across the resistor whichvaries with the plate current and since the volt- Y mitter ill totransmit black and then adjust the oscillator I6 by means of-condenser23 to 100 kc. Then if I transmit white, and turn the gain of theamplifier up, I can thus adjust the intensity of the signal in thereactance tube so that the frequency in the variable frequencyoscillator I6 is increased by 1500 cycles, causing the output from thatoscillator to be 101.5 kc. An indicator is provided at 42 to enable thedetermination of age on anode 26 of the oscillator tube i6 is suppliedthrough this same resistor, the plate voltage on 26 will vary directlyas the plate current drawn in the plate circuit of the reactance tubeand this will also cause the frequency of the oscillator circuit tovary. Therefore, I have several conditions in the oscillator circuitacting in conjunction to vary its frequency over a considerable range.One being the change in the reactance of the inductive component and theother being the change in the plate voltage. The fact that these twoeffects are in the proper phase to increase the frequency swing and thatthe reactance tube is directly coupled to the oscillator materiallyincreases the swing over that which could be obtained with ordinaryreactance modulation.

The fixed frequency oscillator I1 is also an electron coupled oscillatorand includes the LC circuit comprising the inductances 3l32 and thecondenser 33. In this case as in the other the grid 36 acts as theanode, the oscillator section includes the grid 36 and the first gridand cathode, this circuit being conventional.

The two oscillators I5 and I1 feed their signals to the grids of thetwin triode tube l6 which acts as a mixer tube, the two plates of whichare connected together. As previously stated the signals in the outputof this tube, according to accepted theories, are the sum of the twofrequencies from oscillators l5 and H, the difference between the twofrequencies and the individual frequencies.

For one particular mode of operation I assume that a frequency swing of1500 cycles is desired, this being in the audible range. Then the fixedfrequency oscillator i'l may be set for 102.5 kc. and the variablefrequency oscillator set for 100 kc.

If then, the variable frequency oscillator I5 is adjusted to provide a100 kc. signal with no signal or picture black on the reactance tube I,

this signal can be raised to 101.5 kc. when pic- .ture white or maximumsignal is applied to the tube It. This enables me to obtain a frequencyswing of 1500 cycles or a signal in the output of the mixer tube whichis the difference between the signals of oscillators I5 and IT. Thisrepresents a changeof 2500 to 1000 cycles. I am therefore enabled toobtain a much greater frequency swing utilizing but a single tube, thanhas ordinarily been practical in this particular frequency band.

With no signal in the rea'ctance tube l4, the output of the mixer tubei6 there would appear 100 kc., 102.5 kc., 2.5 kc. and 202.5 kc. Then ifthis adjustment. I now have a condition where the frequency will swingbetween and 101.5 kc. or 1500 cycles. And this will cause the lowfrequency output in the mixer to vary between 2500 and 1000 cycles withblack being 2500 cycles and white being'1000 cycles and any frequencybetween the two being diiferent degrees of blackness and whiteness.

Since I am only interested in a varying audible frequency I next passthe signal from the tube l6 through a band pass filter 40; here allsignals but the 1000 to 2500 cycle varyin frequency signal are filteredout and the output from the'filter is a frequency modulated signal whichvaries between 1000.cycles and 2500 cycles depending upon the reflectiveproperties of the picture. This signal may then be used to modulate anytype of transmitter indicated at 4 I It will be noted that I havedesignated the radio transmitter as being relatively close to theconverter and picture transmitter. However, it should be particularlynoted thatthis signal is of such a frequency that it can be readilytransmitted from the converter at one point such as a newspaper ofllceto the transmitter at a remote point over ordinary telephone or landwires.

The transmitter can be a frequency modulated or an amplitude modulatedtransmitter and the :ignal from the converter used to modulate either AsI previously stated the converter could also be built integral with themodulation part of the picture transmitter. As will appear from myaforementioned patent, the transmitter there includes a-photo cell. Theoutput of the photo cell is substantially the same, except formagnitude, as the signal from the filter l3. Therefore, if desired, the1800 cycle oscillator of the transmitter could be eliminated and thesignal from the phOtO cell amplified and applied directly to thereactance tube It. I would then have a straight frequency modulatedapparatus and the demodulation step would be eliminated.

The receiving end of the system is best shown in Fig. 2. Here thesignals, if the transmission is by radio, are received over a suitablereceiver 50. Connected to the receiver 50 is an amplifier in which thesignal is raised to ahigh value. At the output to the receiver thesignal may contain certain undesirable components such as noise andstatic that may have been picked up during transmission, and in additionthe signal may be subject to severe fading. In other respects, however,the signal may be the same as it was when it was used to modulate thetransmitter ll; 1. e. a

frequency modulated signal having a swing between 1000 and 2500 cycles.

Next the signal is put through what I prefer to term a push-pull-cascadelimiter circuit. This is effected .ly feeding the signal into theprimary of a push-pull transformer 52, the secondary of which has itsopposite ends connected through the resistances 53 and 58' to the grids54 and 54' separated. Asssuming that a fairly strong sig-' nal is beingreceived, this signal is amplified to a high level and is applied to thegrid 54 of the first triode section of the tube 55 through a seriesresistance 53. The grid 54 is driven positive on the positive half ofthe signal which causes grid current to flow in the grid-cathodecircuit. This grid current causes an IR drop across the resistor 53 withthe result that the signal into the tube is limited.

The anode voltage for the first triode section is supplied by the mannerin which the second triode section is connected. The anode 51' of thesecond triode is connected to the positive plate voltage supply andnormally this tube draws current from the plate circuit to the cathode59 which is connected through a resistor 58 to the grid 60 of the secondsection and the anode 51 of the first triode section. The circuit thenfor the cathode of the second section is through the first triodesection to the cathode 58 and thence to ground. The voltage drop acrossresistor 58 provides bias for the grid 60. Since the cathode 55 of thefirst section is at ground potential the plate voltage at 51 is aboveground potential due to the current flow in 58.

When the positive signal appears on the grid 54 the plate draws currentwhich is through the resistor 58, thus increasing the negative bias onthe grid 80 which action decreases the normal plate current in thesecond section thus reducing the plate voltage at 51.

, This still further increases the limiting action of the circuit. Thislimiting-action is relatively large, depending, of course, On theamplitude of the signal. The final result is that the resultant waveform is devoid of all amplitude modulations caused by static, noise orother amplitude picked up previously in the system and is substantiallya square wave. Of course, on the other half of the wave the otherlimiter tube 55 is operating in a like manner.

The maximum limiting of the signal voltage may be as much as from 2000to 2. In actual practice I have found that a desirable method ofoperation is to raise the signal to a relatively high value before it islimited thus providing sufiicient signal to always provide limitingaction. In practice I have found it desirable for the limiter to startfunctioning on voltages as low as 2 volts.

As a result of the limiting action, the output from the limiter issubstantially a constant voltage of varying frequency.

The output from the two limiter circuits are connected in push-pull tothe primary of the transformer iii the secondary of which feeds into asloping filter 82. This filter, as the name implies, has a 45 slopingcharacteristic. A signal having a frequency of 1000 cycles, which may bepicture white, will pass through the filter without any substantialloss. As the frequency of the signal increases the loss becomes greater,hence a 2500 cycle signal suffers a very substantial loss. Likewise, thesignals between 1000 and 2500 cycles suffer a loss substantially indirect proportion to their frequencies. The result is that the signal inthe output of the sloping filter 52 comprises a signal that varies inamplitude in a manner corresponding to the original amplitude modulationcomponent. The signal at this point comprises a hybrid signal whichconsists of frequency and amplitude modulations.

The signal is then fed through the transformer 53 the secondary of whichis connected to the amplifier tube 64 which may beanother twin triodewith the two sections connected in parallel, and in which the signal isamplified.

The output from the tube 84 is connected to the transformer 55, thesecondary of which is connected to the anodes of the demodulator tube61. This tube rectifies both halves of the signal which then appears onthe cathodes of the tube which are connected together, and is apulsating direct current the amplitude of which varies according to themodulation envelope. The pulsations in the signal are removed in thefilter comprising the input condenser 58, the choke 58 and followed bythe series choke and condenser 'I0'II connected between the conductor 12and ground. Thus at 13 there remains only a signal which correspondsexactly to the modulation envelope of. the original signal.

The next step in the procedure is to use this signal energy toreconstruct an 1800 cycle amplitude modulated signal in order that itmay be supplied to a conventional facsimile receiver and there used toreproduce the picture.

For generating the 1800- cycle carrier signal, I provide an 1800 cycleaudio oscillator which may be of any standard typ ving a good sine waveand having a constant frequency output of 1800 cycles. The output fromthe'oscillator is connected in push-pull to the grids 8i of the tube 82,which may be a twin triode. The anode voltage for this tube is obtainedfrom the signal (at 13) after it leaves the filter, the signal beingsupplied to the anodes through resonant filters each of which comprisesan inductance 84 and paralleled by capacities 85 and 81 respectively.The two inductances are connected end to end and as stated each has acondenser across it. The signal is fed to the junction of theinductances and capacities. nect to the anodes of the tube 82. The tworesonant filters 84-85 and 88-81 are tuned to the frequency of theoscillator 80. Tube 82, which may be termed a restorer tube, onlyoperates to pass a signal when plate voltage is supplied to the anodes.Because the plate voltages on this tube are being supplied by the signalwhich is substantially a replica of the original signal used to modulatethe carrier at the facsimile transmitter, the signal in the platecircuit of the tube varies with the plate voltage. The signal as itappears across the parallel resonant circuits is an 1800 cycle amplitudemodulated carrier which is supplied through the coupling condensers'90to the potentiometers 9! which are connected in series and grounded attheir junction. The sliders 92 of the potentiometers are connected tothe control grids 83 of the twin 75 triode tube 84 which "operates inpush-pull. The

The leads 88 and 89 con-' cathodes of the tube are resistor 95. Theplate circuit of the tube is connected to the output transformer 96. Thetube 94 amplifies the signal which is thus supplied to the transformer96. The level of the signal may be determined by a meter M indicatordisposed in the output. The output from the inverter may be connecteddirectly to a standard picture receiver for reproduction or it may besent over land wires to the place where the picture receiver is located.

It should particularly be noted that the filter grounded through the Inetwork ahead of the point 13 and comprising the inductance 69 andcapacity 68, and the inductance l6 and capacity?! are effective toprevent anything but the D. C. component of the desired signal fromgetting through to the point 13, and that the resonant circuits 84-85and 8687 are resonant to 1800 and therefore present a high impedance tothe 1800 cycle carrier and therefore readily transfer the 1800 frequencythrough the coupling condensers 90 to the input circuit of the tube 93.However, these resonant circuits readily admit the D. C. energy from 13to the anodes of tube 82.

Thus the signal in the restorer circuit is a clean cut signal withoutany spurious modulations due to the undesirable elements in the incomingsignal.

The two taps shown for the output meter provides means whereby when agreater resistance is in series, the gain can be raised and the properlevel determined for sending the picture by land wire. If the picture isfed directly to the picture receiver. the gain may be lowered and theproper level determined by shifting the meter to the tap where thelesser resistance is in the circuit. In either case the meter enablesthe proper output to be determined.

Having thus described my invention, I am aware that numerous andextensive departures may be made therefrom without departing from thespirit or scope of the invention.

I claim:

1. A system of frequency modulation including a variable frequencyoscillator circuit, a fixed frequency oscillator circuit, a mixedcircuit, said oscillator circuits having their outputs connected to themixer circuit, said variable frequency oscillator circuit including avacuum tube having at least two grids, a cathode and a plate, a resonantcircuit comprising a split inductance and a tuning capacity disposedacross said inductance, one end of said resonant circuit being connecteddirectly to one of said grids to act as an anode and to the other gridacting as a control grid, a reactance tube having at least a grid, acathode and an anode and connected to the resonant circuit to controlthe frequency of the resonant circuit and means for supplying a signalto said reactance tube grid, anode voltage supply means for thereactance tube and oscillator tube serially connected through aresistance to the junction of said split inductance.

2. A system of frequency modulation including a variable frequencyoscillator circuit, a fixed frequency oscillator circuit, a mixercircuit connected to said oscillator circuits, said variable frequencyoscillator circuit including a vacuum tube having at least two grids, acathode and a plate, a resonant circuit comprising an inductance andmeans for tuning said resonant inductance, one end of said resonantcircuit being connected directly to one of said rids, said grid adaptedto act as an anode and the other grid functioning as a control grid, areactance tube having at least a grid, a cathode and an anode connectedto control the frequency of said resonant circuit and means forsupplying a signal to the input of said reactance tube, anode voltagesupply means for said reactance and oscillator tubes serially connectedand voltage dropping resistance means in said supply means connected tovary the anode voltage on said variable oscillator with varying currentdemands in the resactance tube.

LOUIS A. THOMPSON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,629,685 Ditcham May 24, 19271,964,375 Wright et al June 26, 1934 2,060,778 Finch Nov. 10, 19362,298,657 Smith et a1. -,Oct. 13, 1942 2,305,842 Case Dec. 22, 19422,342,943 Kell Feb. 29, 1944 2,403,358 Gerhard et a1 July 2, 1946

